Various industries require the ability to place thin coatings of polymeric materials onto selected substrates. One such industry is the electronics industry, especially the portion thereof which is concerned with the manufacture of polymer monolithic capacitors. Other industries which rely on the production of thin polymeric coatings on various substrates include magnetic tape manufacturers and producers of packaging films.
Capacitors are used in a wide variety of electrical circuits, for example, in relatively high voltage AC power systems (such as the common 110-volt systems) and in relatively low voltage (e.g., under 50 volts) DC systems frequently encountered in printed circuits and the like. Important factors which must be considered in the manufacture of such capacitors are volumetric efficiency, temperature of operation, dissipation factor, especially in AC systems, and behavior upon failure.
The development of electronic devices and circuits of reduced size has led to a need for significantly smaller capacitors having increased volumetric efficiency, or capacitance per unit volume. The polymer monolithic capacitor has been used for such applications.
A monolithic capacitor is one in which the layers of electrodes and dielectric are bonded together in a unitary structure as opposed, for example, to a metallized film capacitor in which self-supporting films are rolled or wound into the capacitor form. A miniaturized capacitor is one of very small dimensions, so as to be suitable for microcircuitry. Small overall size could denote low capacitance of little practical value, except that the thickness of the intervening dielectric layer inversely affects the capacitance between adjacent electrodes, and the number of electrode pairs and dielectric constant of the dielectric directly affects capacitance. Therefore, as a matter of basic capacitor theory, a capacitor having very thin dielectric layers, and many pairs of electrodes or a given capacitor with a dielectric having a high dielectric constant could have substantial capacitance despite being of miniature size with the active area of the electrodes being quite small.
One such type of polymer monolithic multi-layer capacitor is described in application Ser. No. 620,647, now abandoned, cross-referenced herein. That capacitor has a capacitively active section, and two electrode joining sections, each separated from the active section by a sloping section. The capacitor includes a first and second set of electrode layers interleaved with one another, each layer of each set having an active area extending through and contributing to the capacitively active section of the capacitor in a stacked and spaced apart relationship with the active areas of all of the other layers. The electrode layers are joined at the margin in stacked electrically contacting relationship and each layer has a sloped portion between its active area and its margin which contributes to a sloped section of the capacitor. A dielectric coating is in contact with and between each adjacent electrode pair. The dielectric coating has a substantially uniform thickness in the capacitively active section and tapers to zero thickness through the sloping section.
The volumetric efficiency of a capacitor, including the monolithic multi-layer capacitor described above, is normally measured in terms of capacitance per unit volume. Generally, high efficiency is desirable, with values of at least about one-tenth (0.1) microfarad per cubic millimeter for a 50 VDC rated unit being preferred.
As noted above, the volumetric efficiency of the capacitor may be increased by reducing the thickness of the dielectric layer and/or by increasing the number of electrode pairs, both of which may have limits depending upon the capacitor type and its end use.
From the foregoing, it is evident that in order to achieve the results which are desired in accordance with the aforementioned application Ser. No. 620,647, a method for depositing a thin uniform monomeric layer on the desired substrate is absolutely essential. Heretofore, it has been known generally in the art that monomeric layers may be deposited upon substrates. For example, U.S. Pat. No. 3,547,683 and the British counterpart thereof, No. 1,168,641 deal with the vapor deposition of a polymerizable or cross-linkable material which has a vapor pressure under standard temperature and pressure conditions of less than 1 Torr. It appears that the concept embodied in such patents is simply to allow a heated container of the material to vaporize the desired polymerizable or cross-linkable material Such an approach suffers from the fact that by maintaining a polymerizable or cross-linkable material at an elevated temperature for any substantial length of time gives rise to the possibility of degradation and/or polymerization of the material, within the container, both of which are undesirable.
U.S. Pat. Nos. 4,121,537 and 4,207,836 are both concerned with the vapor deposition of a layer of a compound consisting of two or more kinds of elements such as Se, Te or As through flash evaporation, as by dropping raw material, bit-by-bit, into a boat heated to a high temperature with the individual bits being vaporized within a short time. Thus, the approach of using flash evaporation is not taught in said patents to relate to monomeric materials as used in the present invention. A problem with such an approach, as taught in said patents, of course, is that as each drop is vaporized, a burst of vapor is produced, followed by an intermittent period of time during which no vapor is present. Under such conditions, it is difficult to achieve a uniform layer of material on the desired substrate. The foregoing is especially true if the source of the vapor and the substrate are moving in relation to each other.
U.S. Pat. No. 4,153,925 is concerned with the use of electron bombardment or ultraviolet radiation of an organic monomer to make a dielectric layer, generally. However, there is no specific teaching regarding the method by which the monomer is placed on the desired substrate. Likewise, U.S. Pat. Nos. 4,277,516; 4,301,765 and 4,378,382 deal with the glow polymerization of monomers, without any specific indication of how one might achieve a desired uniform monomeric layer. It is believed that the monomers which are glow polymerized and to which those patents relate, are monomers which are typically present under standard temperature and pressure conditions as a gas and thus do not present the same problem as do monomers which are typically used as liquids.
Thus, especially in view of the process disclosed in the aforementioned application, Ser. No. 620,647, now abandoned, there exists a need for a method by which polymerizable and/or cross-linkable materials may be uniformly deposited upon a desired substrate in a controlled manner such that said materials may be subsequently cured to form a desired polymeric layer on said substrate. More generally, a need exists for a method by which materials may be quickly and uniformly deposited upon a desired substrate.